The present invention relates to a process for separation of N-protected-.alpha.-L-aspartyl-L-phenylalanine.
.alpha.-L-aspartyl-L-phenylalanine methyl ester (aspartame) is known as an excellent sweetening agent, and various processes for preparing it have been proposed. However, since .alpha.-L-aspartyl-D-phenylalanine methyl ester which is a diastereomer thereof has no sweetness, no attempt has ever been reported on the use of D,L-phenylalanine as phenylalanine component in the preparation of aspartame and related compounds thereof. In the known processes, for example, in the processes for producing N-formyl-.alpha.-L-aspartyl-L-phenylalanine by condensing N-formyl-L-aspartic anhydride and phenylalanine in acetic acid (U.S. Pat. No. 3,933,781) or in an alkaline water medium (EP-A-No. 0186378), L-phenylalanine is used as phenylalanine component.
If it is made possible to use D,L-phenylalanine which is less expensive than L-phenylalanine for the preparation of aspartame, the preparation process will become more advantageous in industrial application since it is unnecessitated to obtain L-phenylalanine by optical resolution of D,L-phenylalanine or by asymmetric synthesis with a complicated procedure. However, when D,L-phenylalanine is condensed with, for example, an N-protected-L-aspartic anhydride, there are produced, in addition to the objective N-protected-.alpha.-L-aspartyl-L-phenylalanine (1), three other isomers, viz. N-protected-.alpha.-L-aspartyl-D-phenylalanine (2), N-protected-.beta.-L-aspartyl-L-phenylalanine (3) and N-protected-.beta.-L-aspartyl-D-phenylalanine (4) as shown below by chemical formulae: ##STR1##
If it is possible to separate the objective N-protected-.alpha.-L-aspartyl-L-phenylalanine alone from a mixture of the four isomers, the separated compound (1) can be converted into .alpha.-L-aspartyl-L-phenylalanine methyl ester by a method comprising deformylation in a methanol/hydrochloric acid solution and successive esterification (U.S. Pat. No. 3,933,781) in the case where the protecting group is a formyl group, and thus an industrially advantageous aspartame preparation process can be provided.
However, as mentioned before, no report has ever been presented on the use of D,L-phenylalanine for the preparation of aspartame and related compounds thereof nor are available any data about the properties, especially solubility, of N-protected-.alpha.-L-aspartyl-D-phenylalanine and N-protected-.beta.-L-aspartyl-D-phenylalanine produced by condensing N-protected-L-aspartic anhydride and D-phenylalanine, and accordingly there has yet been established no industrial process for selectively separating the desired N-protected-.alpha.-L-aspartyl-L-phenylalanine from a mixture of the four isomers.
In the prior art, the removal of the .beta.-isomer (N-protected-.beta.-L-aspartyl-L-phenylalanine) formed as a by-product when reacting N-protected-L-aspartic anhydride and L-phenylalanine, for example, in the case where the protecting group was a formyl group, has been carried out by separately precipitating the objective N-formyl-.alpha.-L-aspartyl-L-phenylalanine from an acetic acid solution (U.S. Pat. No. 3,933,781) or from an aqueous solution (EP-A-No. 0186378), leaving N-formyl-.beta.-L-aspartyl-L-phenylalanine in the mother liquor. This removal of the .beta.-isomer owes to the greater amount of formation and lower solubility of .alpha.-isomer than .beta.-isomer. Regarding the amount of formation, that of N-formyl-.beta.-L-aspartyl-L-phenylalanine is only about 1/3 to 1/4 of that of N-formyl-.alpha.-L-aspartyl-L-phenylalanine in these methods.
On the other hand, in the case of using D,L-phenylalanine, the amount of N-protected-.alpha.-L-aspartyl-D-phenyalanine produced is almost equal to that of N-protected-.alpha.-L-aspartyl-L-phenylalanine, so that the removal of this isomer becomes the greatest problem.
As a result of studies for solving such problem, the present inventors found that N-protected-.alpha.-L-aspartyl-L-phenylalanine is less soluble in acetic solvents or aqueous solvents than N-protected-.alpha.-L-aspartyl-D-phenylalanine, so that the former alone can be precipitated out by a crystallization. It was further found that when N-protected-L-aspartic anhydride and D,L-phenylalanine are condensed in an acetic solvent or an aqueous solvent, the amount of N-protected-.beta.-L-aspartyl-D-phenylalanine produced is as small as that of N-protected-.beta.-L-aspartyl-L-phenylalanine and these two by-products are not precipitated in a crystallization operation, so that it is possible to separately crystallize N-protected-.alpha.-L-aspartyl-L-phenylalanine alone from a mixture of the four isomers. The present invention was attained on the basis of these findings.